1. Field of the Invention
The present invention relates to a surface-treated ultrafine metal powder for use in a conductive paste filler or in the internal electrode of a multilayer ceramic capacitor, to a method for producing the same, to the conductive metal paste, and to the multilayer ceramic capacitor.
2. Description of the Related Art
An ultrafine metal powder used in the internal electrode of a multilayer ceramic capacitor is a high-purity metal powder having an average particle diameter of, e.g., 0.1 to 1.0 μm and a generally spherical particle configuration. The ultrafine metal powder is produced by a chemical vapor phase reaction, changed into a paste with a binder such as an organic resin added thereto, and then used. The average particle diameter used herein is a volume/area average diameter (d3) (that is, the ratio indicated by (total volume)/(total area) of particles) in a number-based distribution.
The paste is coated in a thin layer onto a ceramic green sheet composed of a ceramic dielectric material by screen printing or the like. A laminated structure consisting of several hundreds of such ceramic green sheets with thin layers coated thereon, i.e., internal electrode layers is subjected to a degreasing step, a sintering step, and a firing step, whereby a multilayer ceramic capacitor is produced.
To implement a recent multilayer ceramic capacitor smaller in size and larger in capacitance, the number of the ceramic green sheets, including the internal electrode layers, should be increased from several hundreds to about one thousand. To complete the technology, the thickness of each of the internal electrode layers should be reduced from 3 μm, which is the thickness of an internal electrode layer used conventionally, to 1.5 μm or less. Accordingly, D90 of the particle size distribution of an ultrafine metal powder as the material of the internal electrode layer is desired to be low correspondingly. Here, D90 indicates a particle diameter corresponding to cumulative 90% in the particle size distribution of an ultra-fine metal powder, which is obtained by measurement and shown in a number-based representation.
In recent years, an ultrafine metal powder having an average particle diameter of about 0.1 to 0.2 μm, which is finer than a conventional ultrafine metal powder having an average particle diameter of about 0.4 μm, has been used. This is because D90 of the particle size distribution of an ultrafine metal powder having a smaller average particle diameter is smaller and therefore D90 of the ultrafine metal powder with an average particle diameter of about 0.1 to 0.2 μm is smaller than D90 of the ultrafine metal powder with an average particle diameter of about 0.4 μm.
In general, an ultrafine metal powder with a small particle diameter exhibits peculiar properties. Because of its particularly active surface state, oxidational expansion due to an oxidation reaction occurs in a low-temperature range (200 to 300° C.) or heat shrinkage is started even in a low-temperature region (400 to 500° C.), which adversely affects electrode formation.
Specifically, an internal electrode using an ultrafine metal powder is cracked owing to oxidational expansion in an oxidizing atmosphere in a degreasing step. Another problem is encountered during the formation of an electrode film in a heating step that the spheroidization under surface tension of the molten metal causes partial increase of thickness of the metal internal electrode. In the latter case, in particular, forming a uniformly thin electrode film becomes difficult and a sufficient large area required for the internal electrode film can not be obtained so that an electric capacitance of the capacitor does not reach an objective value. Since the thick portion is formed through the coagulation of ambient metal, the electrode film is partially ruptured so that the coverage area of the electrode (the area of the portion of the ceramic sheet covered with the electrode film) is reduced. If the film is significantly increased in thickness, the ruptured portions of the film are scattered in a dotted island configuration. The ruptured portions of the film are electrically insulated so that they are non-functional as the electrode film. When the partially thick portion of the film has a projecting configuration, the projecting portion penetrates through the ceramic sheet layer so that the product is formed defective as the electrodes are short-circuited. Even when the projecting portion does not penetrate through the ceramic sheet layer, the inter-electrode distance (distance between neighboring electrodes) is partially reduced so that an increase in current density occurs to cause a degraded lifetime of a multilayer ceramic capacitor.
To prevent the above problems, it has been a conventional practice to strictly control temperature and oxygen concentration in the degreasing step and raise the starting temperature of shrinkage by adding a material similar in composition to the ceramic dielectric material to the metal paste.
When an ultrafine metal powder with an average particle diameter of about 0.1 to 0.2 μm, which is smaller than that of the conventional ultrafine metal powder, begins to be used, however, the foregoing methods commonly practiced against the problems (such as the control of oxygen concentration and temperature and the addition of a material similar in composition to the ceramic dielectric material) are no more effective so that improvements in the oxidation characteristic and heat shrinkage characteristic of the ultrafine metal powder are required.
There has been proposed a technique for reducing defects such as delamination and a crack caused by difference in amounts of heat shrinkage between a ceramic base material and an internal electrode material in heating a multilayer ceramic capacitor. For example, Japanese Laid-Open Patent Publication No. HEI 11-343501 discloses the technique pertaining to a composite Ni fine powder in which an oxide such as TiO2, MnO2, or Cr2O3 is present on the surfaces of Ni particles that have been oxidized through surface-treatment. The fine powder is produced by a wet treatment method including an addition of an aqueous solution containing a metal salt to a slurry of the fine powder and a pH adjustment thereafter using an acid or alkali.
Even with the foregoing technique, it is impossible to suppress an increase in the thickness of the metal internal electrode resulting from the oxidational expansion of the ultrafine metal powder in an oxidizing atmosphere in the degreasing step and from the spheroidization of the molten metal under surface tension during the formation of the electrode film in the firing step.
In the conventional process, there has been demanded an ultrafine metal powder of which the oxidational expansion in the degreasing step and the spheroidization of the molten metal under surface tension during the formation of the electrode film in the firing step have been suppressed during the production of a multilayer ceramic capacitor. However, there has been no ultrafine metal powder which satisfies the demand in either of the cases where the average particle diameter thereof is about 0.4 μm or where the average particle diameter thereof is about 0.1 to 0.2 μm.